serial: fixup /proc/tty/driver/serial after proc_fops conversion

"struct tty_driver *" lies in m->private not in v which is
SEQ_TOKEN_START which is 1 which is enough to trigger NULL dereference
next line:

BUG: unable to handle kernel NULL pointer dereference at 000000ad
IP: [<c040d689>] uart_proc_show+0xe/0x2b0

Noticed by Linus.

Signed-off-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/serial/serial_core.c

···
 
 static int uart_proc_show(struct seq_file *m, void *v)
 {
-	struct tty_driver *ttydrv = v;
+	struct tty_driver *ttydrv = m->private;
 	struct uart_driver *drv = ttydrv->driver_state;
 	int i;